Applicants"" invention relates generally to radiocommunication systems, e.g., cellular or satellite systems and, more particularly, to techniques for supporting and enhancing emergency calling procedures in such systems.
The growth of commercial radiocommunications and, in particular, the explosive growth of cellular radiotelephone systems have changed the ways in which people communicate. One survey indicates that about 80% of the people who purchase mobile communication units and service subscriptions do so to enhance their personal security. Presumably, many of these subscribers would expect to use their mobile units to aid them in urgent situations, e.g., when their vehicle has become disabled or in an emergency situation requiring rapid medical and/or police response. In these circumstances it would be desirable that the radiocommunication system be able to independently determine a location of the mobile unit, particularly in the case where the subscriber does not know his or her precise location. Moreover, it is expected that the FCC will soon require that network operators forward the position of an emergency caller to the emergency service provider.
There are many techniques available to generate mobile unit location information. In a first category, the mobile unit could estimate its own position and send a message with its coordinates when placing an emergency call. This could be accomplished by, for example, providing the mobile unit with a Global Positioning System (GPS) receiver that receives location information from the GPS satellite network. The mobile unit can then transmit this information to the system, which would then forward it to the emergency service provider. This requires, however, significant modification of existing mobile units to include GPS receivers, as well as additional signalling between the mobile units and base stations.
Alternatively, the base stations which transmit signals to, and receive signals from, the mobile units could be used to determine the mobile unit""s location. Various techniques, including attenuation of a mobile unit""s signal, angle-of-arrival, and difference between the time-of-arrival (TDOA) of a mobile unit""s signal at different base stations, have been suggested for usage in providing mobile unit location information. See, for example, the article entitled xe2x80x9cTime Difference of Arrival Technology for Locating Narrowband Cellular Signalsxe2x80x9d by Louis A. Stilp, SPIE Vol. 2602, pp. 134-144. These solutions also have their drawbacks including the need to modify the many existing base stations, e.g., to provide array antennas to support angle-of-arrival techniques or to synchronize base station transmissions to support TDOA techniques.
A third category of strategies for locating mobile units in radiocommunication systems involves the provision of an adjunct system, i.e., a system which may be completely independent of the radiocommunication system or which may share various components (e.g., an antenna) with the radiocommunication system but which processes signals separately therefrom. This may be advantageous, for example, as an expedient solution to providing mobile unit location without modifying the large number of existing base stations in a system. For example, consider the equipment illustrated in FIG. 1 wherein the adjunct scanning units are not co-located with the base stations of radiocommunication system. Therein, a base station 1 supports radiocommunication within cell 2 and, in particular with mobile unit 3. An adjunct system, partially shown by way of scanning units 4, 5 and 6, monitors accesses to the system by mobile unit 3. When mobile unit 3 makes an emergency access, adjunct units 4, 5 and 6 can detect this emergency access by, for example, the presence of a set emergency flag in an origination message or based upon the dialed number. The adjunct units can then use the mobile unit""s transmissions on either a control channel or a traffic channel to provide information to a location processing center 7. The location processing center then uses the information provided by the various adjunct units to, for example, triangulate the position of mobile unit 3 and report this position to an emergency service center 8. More details regarding exemplary usages of adjunct systems can be found in U.S. Pat. No. 5,327,144 to Stilp et al., entitled xe2x80x9cCellular Telephone Location Systemxe2x80x9d, the disclosure of which is incorporated here by reference.
From a system perspective, the operation of the adjunct system becomes more complicated in terms of assigning monitoring duties to specific adjunct receivers. Consider the exemplary system of FIG. 2. Therein, for each cell C1 to C10, there is a respective base station B1 to B10 which is connected to a PSTN (not shown) via a mobile switching center. FIG. 2 also illustrates ten mobile stations M1 to M10, which are movable within a cell and from one cell to another cell. Of course most systems would have more mobile stations than ten. Lastly, a plurality of adjunct receivers may be provided to locate mobile units M1-M10, only seven of which (A1 to A7) are illustrated to simplify the figure.
Each cellular system is assigned a particular frequency band over which it can operate. A set of communication channels is allocated to each cell. For example, between 10 and 30 different voice channels and 1 control channel may be allocated to any given cell. Different sets of communication channels are allocated to neighboring cells since, in order to maintain full radio coverage, cells overlap each other. Using the same channels in adjacent cells would cause cochannel interference in these overlapping areas.
The question arises as to which channels each adjunct station, e.g., A1-A7, should monitor and how each adjunct station should make this determination. As seen in FIG. 2, it may be desirable to have adjunct stations A1 and A2 monitor the transmissions from mobile station M7 due to their proximity thereto even though these adjunct stations are outside of cell C1. Accordingly, it would be desirable to provide techniques for assigning monitoring duties (i.e., determining which control and/or traffic channels each adjunct receiver should monitor) to individual adjunct receivers.
According to one exemplary embodiment of the present invention, an adjunct receiver monitors downlink channels according to a frequency plan associated with the radiocommunication system that it is to monitor. For each channel on which the adjunct receiver receives signals, the adjunct receiver evaluates the received information to determine if it is associated with a control channel or a traffic channel.
The identified control channels are forwarded to a hub or central processing center. The hub receives similar information from a plurality of adjunct receivers and then responds by assigning each adjunct receiver to monitor certain control channels. According to other exemplary embodiments, the adjunct receiver begins this process by monitoring uplink channels.
Once assigned, adjunct receivers begin monitoring their assigned control channels to look for emergency calls made by mobile units. The adjunct units may first synchronize to the downlink control channel to narrow the timing window for receiving uplink bursts. The monitoring of uplink bursts may include, for example, evaluating bursts on a subchannel-by-subchannel basis to identify accesses by different users.
Some adjunct units may miss access bursts due to, for example, changing RF interference conditions. According to exemplary embodiments of the present invention, other adjunct units may broadcast received mobile unit transmission information over a common communication bus so that adjunct units which miss certain information, e.g., traffic channel assignments, still have an opportunity to provide location information to an emergency service center.
Other exemplary embodiments of the present invention describe additional uses of the adjunct system. For example, the adjunct system can also process the signals received by the adjunct receivers to provide a second speech path to the emergency service center. Since the adjunct system receives multiple copies of each emergency access, diversity techniques can be used to improve the second speech path.